This proposal aims at elucidating the underlying molecular mechanisms that lead to drug resistance to PI3K/mTOR inhibitors in breast cancer. The PI3K/mTOR pathway is one of the most commonly activated pathways in breast cancer (>70%), and provides an attractive target for therapeutic intervention. There has been an increasing number of inhibitors developed to target this pathway. Unfortunately, the results from the clinic have been somewhat disappointing, showing only little efficacy for these inhibitors acting as single agents. This is likely due to activation of compensatory pathways that can circumvent the effect of the inhibitors, leading to drug resistance. Therefore, we have focused our efforts on the identification of mechanisms of resistance to PI3K/mTOR inhibitors. The development of efficient drug combination strategies targeting this resistance would benefit a large number of breast cancer patients. Our previous studies showed that contact with the extracellular matrix provides cancer cells with critical cues that permit them to survive treatment with PI3K/mTOR inhibitors, whereas, cells without matrix contact remained sensitive. This matrix-dependent drug resistance was due to adaptive responses that the drug treatment elicited in cancer cells. This adaptive response included up-regulation of several survival pathways and was orchestrated in part by FOXO transcription factors and by regulation of protein synthesis. This work identified a critical role for the regulation of protein synthesis in mediating drug resistance, and, here in this proposal, we plan to expand this important, albeit little explored, aspect of the resistance mechanism to PI3K/mTOR inhibitors. These observations will form the foundation of this proposal. The elucidation of the exact molecular mechanism underlying the matrix-mediated drug resistance will provide the basis for a rational approach to the development of effective combinatorial therapies. This proposal outlines in vitro and in vivo lines of investigation to identify the basis of matrix-mediated drug resistance. In thi proposal I will: 1) identify molecules and mechanisms directly linked to the matrix-adhesions that contribute to the initiation of the adaptive response, 2) investigate how matrix-adhesion regulates protein synthesis and how this contributes to drug resistance, 3) identify tumor-specific molecules by comparing drug-induced adaptive responses in tumors and normal tissues, and 4) utilize these data to design and test novel tumor-specific combination therapeutics to overcome matrix-mediated drug resistance while minimizing adverse effects in normal tissues. The success of these studies is directly linked to the proposed training activities I intend to undertake during the mentored phase of this award. The mentored phase will allow me to deepen my understanding of tumor biology, concentrating on breast cancer, and to obtain advanced training in a highly supportive and innovative training environment of Harvard Medical School and the affiliated hospitals. In addition, to support me in my training, and in my transitio to the independent phase of my career, I have assembled an advisory committee of leading experts in the fields I propose studying. This advisory committee includes my mentor Dr. Joan Brugge (Harvard Medical School), Dr. John Blenis (Harvard Medical School), Dr. Kornelia Polyak (Dana Farber Cancer Institute) and Dr. David Sabatini (Whitehead/MIT). The skills and knowledge acquired during the mentored phase of this award will be instrumental for the above proposed studies and future studies, and for successfully launching my career as an independent investigator.